Influence of microalloying additions on thickness of grain boundary carbides in ferrite–pearlite steels

Abstract

For a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min^?1 through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the ‘tails’ to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease in the transformation temperature and leads to finer carbides. The pearlite nose transformation temperature must be ≦600°C to produce fine (≦0·2 μm) carbides. The austenite grain size, which controls the pearlite colony size, is also very important in determining the thickness of carbides, since the finer the grain size, the greater the carbide density and,for a given amount of C available for precipitation, the finer the resulting carbides. Faster cooling or a higher Mn content refine the pearlite colony size leading to finer carbides. Compared with C–Mn–Al steels, Nb and Ti microalloying additions result in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coarser carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behaviour are discussed.

MST/1858     

Review: Interaction of precipitation with recrystallisation and phase transformation in low alloy steels

Abstract

The processes of precipitation, restoration and phase transformation can interact in complex ways during thermomechanical processing of microalloyed steels to profoundly alter their structures and properties. Precipitation in austenite during hot deformation can strongly modify the kinetics of recovery and recrystallisation, subsequently affecting the nucleation and growth of ferrite during cooling. For steels containing strong carbide/nitride formers, interphase precipitation (IP) can occur in ferrite at the austenite/ferrite interface, conferring significant coherency strengthening. Much of what is known about this phenomenon is attributable to the impressive research efforts of Robert Honeycombe and his colleagues at Cambridge.     

Effect of heating rate on reaustenitisation of low carbon niobium microalloyed steel

Abstract

Austenite formation during a continuous heating in a low carbon niobium microalloyed steel with a pearlite and ferrite initial microstructure has been studied. Characteristic transformation temperatures, Ac ₁, Ac _θ and Ac ₃ and the evolution of austenite formation have been determined by combining dilatometry and metallography in a range of heating rates from 0˙05 to 10 K s^–1. It has been observed that nucleation and growth of austenite depends highly on the applied heating rate. At low heating rates (0˙05 K s^–1) nucleation of austenite takes place both at pearlite nodules and at ferrite grain boundaries, while for higher heating rates (≥0˙5 K s^–1), nucleation at grain boundaries is barely present compared to the nucleation at pearlite nodules. The heating rate also affects the austenite growth path and morphology and, thus, the distribution of martensite in the dual phase microstructure obtained at room temperature.     

Cellular automaton simulation of microstructure evolution during austenite decomposition under continuous cooling conditions

A two-dimensional diffusion based model is developed to describe transformation of austenite into ferrite and pearlite under continuous cooling conditions. The nucleation of ferrite is assumed to occur over grain boundaries and the nucleation of pearlite is assumed to be taking place all over the grain and at growing ferrite-austenite interfaces, when the composition and temperature conditions are favourable. A cellular automaton algorithm, with transformation rules based on this model is used for the growth of ferrite and pearlite. Model predicted results for continuous cooling transformations are verified by comparing the model predicted microstructure features with the experimental measurements for two sets of plain carbon steels of different composition and austenite grain size. Using the model, it is possible to generate results like under-cooling to start ferrite and pearlite transformations, which are difficult to obtain experimentally.     

The interaction between proeutectoid ferrite and austenite during the isothermal transformation of two low-carbon steels — a new model for the decomposition of austenite

Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been employed to examine the austenite to proeutectoid ferrite and ferrite/carbide reactions in two low-carbon (0.04 wt%) steels. It is demonstrated that proeutectoid ferrite (both polygonal and Widmanstätten) can partition the prior austenite grains into several smaller units or pools. It is also shown that prior to the initiation of the pearlite reaction, ferrite grain growth can occur. The pools of austenite exert a Zener-like drag force on the migrating ferrite grain boundaries. However, the ferrite boundaries can eventually break away and small pools of austenite become completely embedded in single proeutectoid ferrite grains. Subsequently, these small pools of austenite transform to discrete regions of cementite, together with epitaxial ferrite. Conversely, certain small pools remain in contact with the ferrite grain boundaries and it is considered that transformation of these latter pools will eventually lead to the formation of massive films of cementite at the ferrite grain boundaries. Larger pools of austenite prevent ferrite boundary breakaway, and these latter, austenitic regions eventually transform to pearlite.     

Effects of simulated on line accelerated cooling processing on transformation temperatures and microstructure in microalloyed steels Part 1 – Strip processing

Abstract

Simulations of industrial thermomechanical processing and on line accelerated cooling of a low carbon microalloyed strip steel were carried out using a quench deformation dilatometer. Effects of processing parameters, such as accelerated cooling rate T and accelerated cooling interrupt temperature T_I on the critical transformation temperatures and final microstructure were determined. The most important on line accelerated cooling (OLAC) processing parameter is the accelerated cooling interrupt temperature, which controls whether the transformed microstructure is predominantly ferrite or bainite. A variety of (Ti, Nb, Fe) carbide, nitride, and carbonitride precipitates are present in the OLAC processed samples. The final precipitate distribution is developed at three stages of processing, namely: dissolution and coarsening of pre-existing precipitates at the reheat temperature, precipitation in deformed austenite during the deformation schedule, and precipitation in ferrite after the interruption of accelerated cooling. Maximum precipitation strengthening occurs for T_I=700–640°C.

MST/3424     

Bainite transformation during continuous cooling of low carbon microalloyed steel

Abstract

The evolution of the final microstructure for a low carbon Nb–Ti microalloyed plate steel was studied during a simulation of thermomechanical processing for hot rolling following by accelerated cooling. The effects of austenite deformation below the non-recrystallisation temperature T _NR, cooling rate, and interrupt temperature on the formation of conventional (intergranular) bainite (CB), acicular ferrite (intragranular) (AF), and martensite–austenite (MA) constituents were determined. With increases in austenite deformation and cooling rate, and decrease in the interrupt temperature, the final microstructure changed from a mixture of CB+MA through CB+AF+MA to a dual phase AF+MA.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

Boundary characterisation of X65 pipeline steel using analytical electron microscopy

Analytical electron microscopy was used to characterise grain boundaries (GBs) and interphase boundaries (IBs) of X65 pipeline steel. There was no segregation of P or S at the proeutectoid ferrite GBs. This indicates that contrary to literature expectations, P and S are unlikely to be involved in the mechanism of SCC of pipeline steels. There was Mn segregation at IBs between pro-eutectoid ferrite and pearlitic cementite, and desegregated from the IBs between pro-eutectoid ferrite and pearlitic ferrite. This pattern of Mn segregation is attributed to diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite. A new mechanism was proposed for pearlite formation. A GB carbide first forms at an α : α GB, and then grows along the α/γ interface. Subsequently pearlite initiates from this interface carbide. This revised version was published online in September 2006 with corrections to the Cover Date.     

Microstructural development in non-oriented lamination steels

The isothermal decomposition of austenite in two commercial low carbon (0.04 w/o) steels has been examined using scanning electron microscopy and transmission electron microscopy. Particular emphasis has been placed on analysing the pearlite reaction and the development of massive films of cementite at pro-eutectoid ferrite/pearlite interfaces. Similarly, grain boundary precipitation of cementite has been investigated. The results strongly support the contention that films of cementite at ferrite/pearlite interfaces form predominantly by a coarsening process. In addition, it is shown that grain boundary precipitation of cementite can occur from super-saturated ferrite or from the decomposition of austenite. Examination of the early stages of the pearlite reaction has provided evidence that multiple nucleation of cementite can be a necessary precursor to the development of a pearlite colony.     

Importance of deformation induced ferrite and factors which control its formation

Abstract

The influence of strain, strain rate, temperature, and grain size on the formation of deformation induced ferrite has been examined. Deformation induced ferrite forms very readily in both fine and coarse grained steels and much more rapidly than the ferrite from strain free austenite. Very small strains are sufficient to induce the production of such ferrite and the temperature range over which it appears spans from just below the Ae₃ temperature down to the undeformed Ar₃ temperature. Although it forms readily in both coarse and fine grained steels, the volume fraction produced is sensitive to the austenite grain size. In coarse grained steels, deformation at low strain rates is concentrated along the grain faces; extensive dynamic recovery occurs, which is why the ferrite remains soft, so that only thin ferrite films are able to form. At higher strain rates, work hardening takes place so that the strength of the ferrite at high strains approaches that of the austenite. Under these conditions, the deformation is propagated towards the centres of the austenite grains and larger volume fractions of deformation induced ferrite are able to form. In fine grained steels, the flow stress in the austenite grain boundary region is increased, so that when ferrite first forms, a considerable amount of work hardening takes place, which strengthens the ferrite. When combined with the increased number of triple points present in the material, the increased work hardening promotes spreading of the deformation, with the result that larger volume fractions of ferrite are produced, even at low strains and strain rates.     

Influence of deformation induced ferrite,grain boundary sliding,and dynamic recrystallisation on hot ductility of 0·1–0·75%C steels

Abstract

The influence of C on hot ductility in the temperature range 600–1000°C has been examined for three C contents (0·1, 0·4, and 0·75 wt-%). Using a strain rate of 3 × 10^?3 s^?1, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 × 10^?2 and 3 × 10^?4 s^?1 were examined. At the strain rate of 3 × 10^?3 s^?1, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the γ–α transformation. Thin films of the softer deformation induced ferrite formed around the γ grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae₃) and dynamic recrystallisation was possible. The thin films of deformation induced ferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the 0·1%C steel corresponded to the presence of a large volume fraction of ferrite, this being the more ductile phase. For the 0·4%C steel decreasing the strain rate to 3 × 10^?4 s^?1 resulted in a very wide trough – extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding in the γ. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation induced ferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae₃, resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·1%C.

MST/1809     

Understanding the low temperature end of the hot ductility trough in steels

Abstract

The low temperature end of the hot ductility trough has been examined for steels which have been solution treated at ～1300°C before tensile testing in the temperature range of 1000–600°C. Failure in the trough in this region is intergranular ductile and occurs by strain intensification in the thin film of ferrite surrounding the prior austenite grain. The strain causes voiding to occur at the inclusions situated at the boundaries, the cavities gradually linking up to give failure. In steels which are solution treated before tensile testing, the depth of the trough is shown to be controlled by the volume fraction of the second phase particles, their size and the separation between the particles. Recovery in ductility on the low temperature side of the trough is solely dependent on being able to produce a sufficiently large quantity of ferrite to prevent strain concentration (～40%). Often this has to await the test temperature falling below the AR ₃ in which case wide troughs are formed. However, if conditions are right, very narrow troughs can be produced in which the ferrite that is formed is deformation induced. The width of the trough at the low temperature end of the trough is shown to decrease with increase in strain rate, refinement of the austenite grain size, increase in cooling rate from the solution treatment temperature, decrease in the volume fraction of sulphides situated at the austenite grain boundaries and reduction in the Mn and C contents. The depth of the trough decreases in a similar manner with all these variables except for C and Mn, where for the former there is no effect and for the latter, increasing the Mn level reduces the depth. Narrow troughs on this side of the trough are dependent on being able to form deformation induced ferrite in sufficiently large amounts so as to improve the ductility at temperatures above the AR ₃. A model is proposed to account for most of these observations.     

Effect of austenite deformation on crystallographic texture during transformations in microalloyed bainitic steel

Abstract

The evolution of the texture of ferrite as a function of the coiling temperature has been studied in a hot rolled Nb alloyed CMnMoCrB complex phase steel by means of electron backscatter diffraction. Coiling that steel at 720 ° C led to ferrite and pearlite, and coiling at 550 ° C produced a bainite-martensite microstructure. The presence of residual austenite in the steels coiled at 680 and 550 ° C allowed for texture measurements in γ. Analyses of texture gave fundamental information on the decomposition of γ in both the recrystallised state and the deformed state. It was found that austenite, initially deformed below the non-recrystallisation temperature T_nr, recrystallised statically d partially during the γ α and the γ ^d α _b transformations. In the specimen coiled at 680 ° C, primary ferrite and bainite could be distinguished based on the confidence indexof the diffraction pattern. A clear variant selection was observed for the γ ^d α _b transformation, as arotation of ? ₁ = 30 ° occurred inthe austenite between the ferrite and the bainite formations. The bainite was found to result mainly from the decomposition of the brass {110} 〈 112 〉 and Goss {110} 〈 001 〉 orientations of deformed austenite. The residual austenite was found to be recrystallised γ γ austenite with the cube{001} 〈 100 〉 orientation. Coiling simulations were performed in a dilatometer starting from different austenite grains sizes and deformation states. In the most deformed specimens, the deformation state of the austenite and the combined effects between the different alloying elements presentin the steel were responsible for a solute drag like effect.     

Effect of Nb-Mo additions on precipitation behaviour in V microalloyed TRIP-assisted steels

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates.     

Relative importance of transformation temperatures and sulphur content on hot ductility of steels

Abstract

Low (0·3%) and high manganese (1·4%) plain C – Mn steels with varying sulphur levels have had their hot ductility determined over the temperature range 700 – 1000°C, both after 'solution treatment' at 1330°C and directly after casting. It has been established that the width, depth and position of the hot ductility curves after solution treatment is more related to the transformation behaviour than either the sulphur in solution or the sulphide volume fraction or distribution. The growth of deformation induced ferrite at the austenite boundaries seems to be mainly diffusion controlled, and the higher is the transformation temperature for the γ – α phase change, the faster is the growth. Large amounts of ferrite can then form, giving good ductility. Thus, high transformation temperatures Ae ₃ or Ar ₃ are required to produce narrow ductility troughs. It is believed that any detrimental influence of the sulphides on these 'solution treated' steels is swamped by the rapid increase in ferrite volume fraction. For the as cast state, as more sulphides are able to precipitate at the interdendritic boundaries and austenite grain boundaries than in the solution treated condition, increasing the sulphur level causes a small deterioration in ductility at the high temperature end of the trough. In the present work, only narrow troughs have been found. This is in contrast to previous work on as cast C – Mn – Nb – Al steels, which exhibited wide troughs in the ductility curves, where it was shown that higher total sulphur levels lead to considerably worse ductility and that sulphur can be as detrimental to the ductility as niobium. It is recommended that, to avoid transverse cracking during continuous casting, in addition to keeping the sulphur level low, the carbon and manganese should also be as low as possible.     

INFLUENCE OF TWO-PHASE REGION ROLLING ON TRANSFORMATION AND RECRYSTALLIZATION BEHAVIOUR OF FERRITE IN COPPER-CONTAINING HSLA STEEL

Hot deformation of copper-containing microalloyed steels in the two-phase region was carried out to study the effect of copper on transformation and recrystallization behaviour of ferrite in HSLA steels. it was found that presence of copper could decrease the austenite to ferrite transformation temperature. The precipitation of epsilon-copper in ferrite could retard recovery and recrystallization by dislocation and grain boundary pinning in deformed ferrite. Retardation of transformation and ferrite recrystallization resulted in a less mixed structure consisting of fine transformed and recrystallized ferrite in the copper-containing steels.     

Relationship between hardness and tensile properties in various single structured steels

Abstract

An attempt has been made to establish a relationship between hardness and tensile properties for various single structured steels: ferrite, pearlite, bainite, and martensite. It is found that the proportionality constant A _Y of hardness to yield strength changes from 5.79 to 3.17 and is highest for the ferrite steel and lowest for the tempered martensitic steels. A less pronounced change was found in the proportionality constant A _T of hardness to tensile strength (from 3.97 to 2.72). A dependence on microstructure of the proportionality constant at 8% strain A _0.08 was found as well. This difference in A was found to be attributable mostly to the effect of different work hardening behaviours owing to different microstructures. Regression analysis shows that hardness can be expressed as a function of accessible material parameters such as composition, grain size, and transformation temperatures for various single structured steels within a certain degree of accuracy.     

Austenite and ferrite grain sizes in interstitial free steel

Abstract

An oxidation method has been employed to reveal prior austenite grain boundaries in C–Mn and interstitial free (IF) steels. The ability of this technique to reveal prior austenite grain boundaries is assessed by comparing its results with those of an etching method applied on the C–Mn steel. Optimum conditions were established by trial and error. The conditions varied with different steels and with heat treatment temperature. In the IF steel rapid grain growth at high temperatures in the ferrite range made a significant contribution to the prevention of grain refinement through transformation. Attempts to obtain the smallest prior austenite grain size in the IF steel to assess the ability of the oxidation technique to reveal fine austenite grains led to an average austenite grain size of 80 μm in warm rolled samples after the shortest holding time at 950°C.

MST/3203     

Dispersion hardening by niobium carbonitride precipitation in ferrite

Abstract

The formation offine carbonitride distributions in an experimental, low carbon, high niobium microalloyed steel transformed at various cooling rates was accompanied by TEM observations, tensile tests, and hardness measurements. Interphase precipitation or carbonitride formation on ferrite dislocations were observed to be the only precipitation modes which were responsible for the nucleation of new particles inferrite. An increase of only ～ 90 MPa in yield strength was found as the result of carbonitride precipitation on dislocations that occurred in an acicular ferrite microstructure with a dislocation density of (3–5) × 10¹⁰ cm^-2. A yield strength contribution of ～ 200 MPa was associated with interphase precipitation in polygonal ferrite formed during slow cooling of undeformed austenite. However, interphase precipitation was encountered only occasionally and did not contribute to the strength in the presence of carbonitride particles nucleated during austenite deformation.